The invention relates generally to cardiac pacers, and more particularly to noninvasively programmable cardiac pacers employing automatic data processing techniques.
The physical characteristics of the human heart lend themselves to various interactive artificial pacing systems. There are two major pumping chambers in the heart, the left and right ventricles. Simultaneously contracting, these chambers expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from the left and right atria, respectively. The atria are smaller antechambers which contract in a separate action which precedes the major ventricular contraction by an interval of about 100 milliseconds (ms), known as the AV delay, approximately one-eighth of the cardiac cycle. The contractions arise from a wave of electrical excitation which begins in the right atrium and spreads to the left atrium. The excitation then enters the atrio-ventricular (AV) node which delays its passage via the bundle of His into the ventricles.
Electrical signals corresponding to the contractions appear in the electrocardiogram. A small signal known as the P-wave accompanies atrial contraction while a much larger signal, known as the QRS complex, with a predominant R-wave, accompanies the ventricular contraction. Ventricular repolarization prior to the next contraction is marked by a small signal in the electrocardiogram known as the T-wave. The P and R waves can be very reliably detected as timing signals by electrical leads in contact with the respective heart chambers.
The typical implanted cardiac pacer operates by supplying missing stimulation pulses on a pacing lead attached to the ventricle. The R-wave can be sensed by the same lead. An additional lead contacts the atrium to sense P-waves, if desired. In AV sequential pacers, discussed below, the atrial lead is also used for atrial stimulation.
One of the problems treated by cardiac pacers is heart block caused by impairment of the ability of the bundle of His to conduct normal excitation from the atrium to the ventricle. It has long been apparent that in treating this form of heart disease it is desirable to base stimulation of the ventricles on the P-wave cycle. This synchronization maintains the heart's normal physiological pacing pattern. Thus, the sino-atrial node, which governs the interval between atrial depolarizations (i.e., the atrial rate) according to the body's needs, controls the artificial ventricular rate in the normal manner.
It is also well known that ventricular stimulation should not be applied during the repolarization period (Q-T) following ventricular contraction for about three-eighths of the cardiac cycle. Stimulation during the Q-T transition can induce undesirable heart rhythms. A spontaneous ventricular beat can arise through normal AV conduction or spuriously as in ectopic ventricular activity. In the latter case, the ventricular beat does not have the normal relationship to atrial excitation.
Various systems for inhibited ventricular stimulation due to spontaneous ventricular signals have been proposed, see for example U.S. Pat. No. 4,386,610 issued June 7, 1983 entitled "Ventricular-Inhibited Cardiac Pacer" by Michael E. Leckrone.
Patients without normal atrial activity, as in symptomatic bradycardia, often have a need for atrial stimulation as well as ventricular stimulation which alone achieves about seventy-five percent of the combined volume flow. AV sequential pacers have been proposed for stimulating the atria and the ventricles. The system in the afore-mentioned patent application, for example, senses and stimulates on both atrial and ventricular leads to provide an atrial-based, AV sequential, ventricular-inhibited pacing mode.
Cardiac pacers are life supporting, theraputic medical devices. They are surgically implanted and remain within a living person's body for years. The vital considerations in cardiac pacing technology tend to dictate a conservative approach, if not reluctance, toward commercially exploiting new developments in electronic circuitry. These tendencies are enhanced by the fact that the relatively simple functional requirements of prior art pacers have been easily implemented using preexisting well-established hardware circuit configurations, the need to avoid excessive heat dissipation, and also by the state of the art in compact batteries which limits current drain to avoid unnecessary replacements which require surgery and reprogramming of an expensive new pacer. The keystone is reliability, followed closely by compactness and low current drain.
In the past, pacers have been implemented by analog or digital timing techniques. Digitally timed pacers having externally programmable pulse parameters have been on the market for several years. For example, the "Omni-Atricor" marketed by the assignee of the present application, Cordis Corporation, employs a reed switch in the implanted pacer which responds to a pulsating magnetic field produced by a magnetic programmer such as Cordis' programmer 222B to program rate, pulse amplitude and other variables. The reed switch is also used to implement a magnet rate mode when a permanent magnet is placed near the pacer causing it to revert to a fixed rate mode in which it will not respond to natural activity.
Stored program data processing devices have been suggested for implants before. See, for example, U.S. Pat. No. 4,424,812 issued Jan. 10, 1984 by Alan Lesnick, entitled "Implantable Externally Programmable Microprocessor-Controlled tissue Stimulator", assigned to the assignee of the present application, which discloses a neural stimulator in which the timing of the pulse rate and pulse width intervals is determined by discrete counter circuits. The programming advantages of single chip general purpose microcomputers are not readily exploitable in the battery-limited cardiac pacer technology of today due to excessive current drain. This is even true of microcomputers based on complementary symmetry metal oxide semiconductor (CMOS) technology such as the RCA CDP 1802 although it does have less current drain.
Microcomputers are, nonetheless, extremely adaptive devices suited by design to making simple as complex logical decisions and taking alternative action. Microprocessor technology presents the challenge of making a pacing routine which monitors sense amplifier outputs indicative of spontaneous activity of the heart and safely determines what type of stimulation would be best suited to the existing condition. It is conceivable that the pacer will diagnose the patient's cardiac function, prescribe the correct stimulation routine and automatically pace the patient's heart accordingly as long as necessary. The chief problem in meeting this challenge is to optimize the software and hardware design as a whole to take advantage of the capabilities of microprocessing while conserving space with a very dense stored program and minimizing current drain in the best practicable way.
The reliability of any digital timing system is keyed to the reliability of the clock circuit which drives it. Crystal oscillators are precise but can have catastrophic failures which must be prevented from producing a life threatening situation. Similarly, weak battery signaling is particularly critical with microprocessors because of their increased current drain. It would also be desirable to treat atrial arrhythmia, but the problem is how to define arrhythmia so that the microprocessor will be able to recognize it, how to treat it when it happens, how to decide when it is over, and how to resume normal pacing.
Ideally, in a microprocessor-based pacer it is desirable to retain the programmability of pacer parameters and to enable preexisting programmers which have already been widely marketed to be used.
One of the problems with pacers which sense on both atrial and ventricular channels is the effect that a stimulation pulse on one channel has on the sense amplifier in the other channel. Ideally the sense amplifier circuit should be designed so that a stimulation pulse on the other channel has no adverse effect on the sense amplifiers.